1. Field of the Invention
This invention relates to a ferrite carrier for two-component electrophotographic developers used in copying machines, printers, and the like.
2. Description of Related Art
A two-component developer used for developing an electrostatic latent image in electrophotography comprises a toner and a carrier. The carrier is mixed and agitated with the toner in a development box to give a desired charge to the toner and carries the charged toner onto an electrostatic latent image formed on a photosensitive material (photoreceptor) to form a toner image.
The carrier remains on the magnet and is returned to the development box where it is again mixed and agitated with fresh toner particles for repeated use.
In order to obtain high image quality over a service life of a developer in a stable manner, the carrier is required to have stable characteristics over the life.
In the recent two-component development system, soft ferrite carriers have been replacing conventionally used oxide-coated iron powder or resin-coated iron powder to provide high quality images and to have a long life. Soft ferrites, typically represented by formula: MO.sub.a M'O.sub.b (Fe.sub.2 O.sub.3)c (wherein M and M' each represent a metal element; and a, b and c are each an integer), include Ni-Zn ferrites, Cu-Zn ferrites, and Cu-Zn-Mg ferrites.
Compared with conventional iron powder carriers, the soft ferrite carriers possess many advantageous characters for securing high image quality and a long life. However, use of such metals as Ni, Cu and Zn has recently come to be avoided under strict environmental restrictions.
While the conventional iron powder or magnetite carriers are environmentally benign, giving no adverse influences to the environment, it is difficult with these carriers to enjoy image quality and a life comparable to those of the above-mentioned ferrite carriers.
In the ferrite carriers proposed to date are also included Li-Mn ferrites as disclosed in Japanese Patent Laid-Open Nos. 215664/83 and 297857/87; Mn-Mg ferrites as disclosed in Japanese Patent Laid-Open Nos. 123552/83 and 111159/84; and Mn-Mg-Sr ferrites as described in Japanese Patent Laid-Open No. 22150/96. Lithium in the Li-Mn ferrites, however, is liable to be affected by the surrounding conditions, such as temperature and humidity, and greatly vary in properties. The state-of-the-art Mn-Mg ferrites are unsatisfactory similarly to the other conventional ferrite carriers in that the problem of reducing variation of magnetization among carrier particles still remains unsolved. An Mn-Mg-Sr ferrite carrier has been proposed as a solution to the above problem but has difficulty in achieving uniformity of surface properties (the degree of grain boundary growth), which causes great variation of characteristics when it is used as coated with a resin.
In recent years a so-called soft development system using a low saturation magnetization carrier has been introduced to obtain images of high quality. In this connection it is difficult with the Mn-Mg-Sr ferrite to stably produce a low saturation magnetization carrier.
Japanese Patent Laid-Open Nos. 227267/85, 200551/86, 297856/87, 297857/87, 110253/94, and 20658/95 propose addition of metals, such as V, As, Bi, Sb, Pb, Cu, B, Sn, Si, Li, and P, or oxides, carbonates or sulfates thereof as a resistivity regulator or a sintering aid for ferrite carriers. However, none of these additives was found effective in reducing variation of magnetization among particles, producing a low saturation magnetization carrier stably, and making the carrier surface uniform.